The use of fluid film damping for high speed rotating shafts in turbomachinery is well known in the art. In general, such a damping system includes a radially movable non-rotating bearing support member positioned generally coaxially with the shaft, a bearing positioned between the movable bearing support member and the rotatable shaft, and a pair of axially spaced O-rings defining an annular fluid film chamber between the movable bearing support member and a fixed housing. The movable bearing support member can be an outer annular race of a roller bearing element or a cage of tilt pad bearings. The annular fluid film chamber is used to confine a pressurized oil film. The clearance between the movable bearing support member and the fixed housing is very small, so that the oil film can be squeezed between the two confronting surfaces. During rotation at high speeds, the shaft may vibrate, moving transversely radially and orbitally, causing the movable bearing support member to also move in the same direction. Such motion acts to exert a compressive force on a portion of the oil film so as to squeeze an annular segment of the oil film, thereby causing viscous flow of the oil and resistance to the motion of the movable bearing support member.
To achieve acceptable damping from a squeeze film damping assembly, the non-rotatable bearing support member must be able to move within the housing. This is hard to achieve when, even though the O-rings are still adequate to horizontally center the movable bearing support member, the O-rings cannot support the weight of the shaft and bearing, thus permitting the movable bearing support member to rest on the bottom of the housing bore.
In Streifert, U.S. Pat. No. 4,027,931, a non-rotating, movable annular bearing support member, which is provided with a fluid film damper, is positioned between a rotating shaft and a stationary bearing housing with the movable bearing support member carrying the bearings which act to support the shaft within the housing. The movable bearing support member is supported by a squirrel cage spring for centering the shaft, the squirrel cage spring being mounted coaxially with the shaft and extending axially relative to the movable bearing support member so that one end of the squirrel cage spring is secured to the stationary bearing housing and the other end of the squirrel cage spring is attached to the movable bearing support member, forming a cantilever support. The fluid film dampening is achieved by positioning two O-ring seals at axially spaced apart locations in the annular space between the inside wall of the stationary bearing housing and the outer wall of the movable bearing support member to provide a squeeze film cavity, and introducing oil under pressure into the squeeze film cavity to form the oil film damper. Such pressure can be the normal oil supply pressure, which is generally in the range of about 15 to about 20 psig.
To improve the effectiveness of the oil film damping system, various mechanisms have been employed including springs for centering the shaft. One such prior art mechanism, which addresses the problem of compensating for the dead weight of the rotor shaft, is disclosed in Streifert, U.S. Pat. No. 4,027,931, wherein at least one helper spring is mounted within the portion of the annular squeeze film cavity below the horizontal centerline of the shaft so as to support the dead weight of the shaft and thus eliminate the harmful effects of the heavy shaft upon the squirrel cage spring and the squeeze film cavity. Each helper spring is a longitudinally extending beam having radially outwardly extending support pads at the axial ends of the radially outer surface of the beam and a radially inwardly extending pad at the center of the radially inner surface of the beam.
Similarly, Marmol et al, U.S. Pat. No. 4,981,415 discloses the use of one or two segmented arcuate springs mounted between a fixed support member and a non-rotatable movable bearing support member, with the damper film being formed between the movable bearing support member and the fixed support member, and with a roller bearing positioned between the movable bearing support member and the shaft. Each segmented arcuate spring consists of five segments positioned to form an annular ring generally coaxial with the shaft, with each segment having a radially outwardly directed land at each end of its radially outer surface and an inwardly directed land at the center of its radially inner surface, whereby when the inwardly directed land contacts the shaft the center portion of the segment is caused to flex radially outwardly. The segmented arcuate springs function to absorb the energy attendant the vibration of the shaft during operation. In the static condition, the springs also serve to center the shaft.
Important to the proper functioning of any of the foregoing described fluid film damper systems is proper attention to the stiffness and damping of the system. Effective damping of vibrational movement of the shaft in a turbomachine is enhanced if uniformity of the damper film is preserved between the movable bearing support member and the fixed housing. The uniformity of the damper film is difficult to achieve in a squeeze film bearing support system for a horizontally extending shaft, since the weight of the shaft places a high unidirectional loading upon the movable bearing support member. As a result of this loading, the movable bearing support member is hindered in its ability to move vertically within the housing, and consequently is unable to respond freely to vertical shaft vibrations. By supporting the movable bearing support member in a manner that reduces the stiffness associated with its radial movement, the movable bearing support member is permitted to respond freely to vertical shaft vibrations as well as to horizontal shaft vibrations, and the uniformity of the damper film can be more effectively maintained.